1. Field of the Invention
This invention relates to clocking for computer systems, and more particularly, to clock generation and redundancy.
2. Description of the Related Art
High availability computer systems such as file servers typically require reliable power and clocking. Power and/or clock failures may result in system down time. In some systems, even a momentary interruption in power or clocking may result in a system crash. In order to prevent system down time, redundancy may be built into the power and/or clocking circuitry.
Redundancy for clocking of a computer system may be provided using multiple clock boards. Each of the clock boards may include detection circuitry for detecting the presence of a clock signal which is provided to the computer system. One clock board may act as a primary clock board, with one or more clock boards acting as secondary clock boards. If the primary clock board fails, one of the secondary clock boards may detect this failure and take over as the primary clock board.
Some clock boards use clock synthesizers in order to generate a clock signal at the necessary frequency and amplitude. Many such clock synthesizers suffer from various drawbacks. Often times, clocking in a computer system cannot be protected from frequency drift that may occur in an output clock signal of a clock synthesizer. If the frequency drifts outside a certain range, system operation may be adversely affected. A clock synthesizer often times cannot be used as a clock follower or as a redundant clock source, and may thus necessitate the need for additional circuitry in computer systems where clocking redundancy is required. Since a clock synthesizer may not be useful for providing clock redundancy on its own, use of a voltage controlled crystal oscillator (VCXO) may be required, which may not be cost effective. Furthermore, additional circuitry may require a higher component count and use up more area of a printed circuit board that may otherwise be used to provide additional functionality.